Mature T-cell Proliferation Factor 1 (MTCP-1) and TCL-1 genes form a novel family that is involved in T-cell malignancies and lymphoid proliferation. TCL-1 and MTCP-1 oncogenes encode for 14kDa proteins that share 41% identical amino acid residues but have no significant similarities with other known proteins. The crystal structure of MTCP-1 will be used to design mutations to study the function, and peptide analogs as potential drugs for treatment of T-cell malignancies. Crystals of MTCP-1 grow in space group P6 with the unit cell dimensions of a = b = 61.41 and c = 87.31[unreadable]. The crystals diffract to 2.5-2.0[unreadable] resolution, three potential heavy atom derivatives have been found, and phases result in a partially interpretable electron density map. There are two problems: the X-ray data are weak and radiation decay reduces the accuracy of the measurements; and the derivatives are not mutually isomorphous. Synchrotron time is requested to obtain accurate high resolution data for the native and anomalous data for the derivatives in order to improve the phases. We are also preparing selenomethionine enriched MTCP-1 for MAD phasing.